As Henry stated, the check valve on the top outlet can be the problem. (Where the red hose comes out) It gets stuck open and will not close up allowing anti freeze to flow back into the HW heater.
Some times a tap on the check will seat it. Other times you have to undo the outlet hose fitting, move it aside and then push the check back by hand.
It is for this one reason the older 2 vavle bypass system worked better. Yes it cost a few bucks more and you had to make sure all valves went the right way but at least it always worked. When I used to do the pink stuff winterize I had this problem often. In my case it would shut after a few quarts went in. Seems minerals get the check stuck open and you never notice a problem until it comes to winterizing.
Good luck and hope this helps
PS here are pic of one of my 2004 Sunlines HW heater setups. Yours is most likley just like this.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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